As the Arc (1998) pointed out, the federal definition of "developmental disabilities" does not include some people that fall under the AAMR definition of "mental retardation". The major difference is in terms of the number of functional limitations. The AAMR definition applies to people with two or more functional limitations, while the DD Act definition applies to people with three or more limitations. The Arc states "87% of people with mental retardation have mild mental retardation and, unless they have a second disability, often do not have limits in three areas or the limits may show up in areas other than those listed in the developmental disabilities definition."
Many states do not adhere to these definitions when they define eligibility for services. Some, for instance, retain the categorical labels. Some have different age limitations. Programs funded under the Developmental Disabilities Act follow the federal definition, but there is considerable variation in the definitions used for service provision. The Connecticut Advisory Commission on Services and Supports for Persons with Developmental Disabilities (2002) surveyed state definitions and identified five rough categories.
- Twenty two (22) the states have a definition of developmental disability that closely mirrors the definition in the Title I, Programs for Individuals with Developmental Disabilities.
- Seventeen (17) states have adopted definitions that appear similar to the Title I definition, however, either specifically exclude individuals with physical impairments only or do not specify that individuals with physical impairments only are eligible for services. Additionally, definitions for this grouping of states do not appear to specify that mental retardation is needed for service eligibility.
- Two states (Colorado and Minnesota) have definitions that at first glance appear to broaden their definition beyond mental retardation only. Nevertheless, they specify that individuals must have general intellectual functioning or adaptive behavior similar to that of a person with mental retardation.
- Three states (Vermont, Maine, and Texas) appear to have definitions that limit services and supports from the principal agency to individuals with mental retardation and autism.
- Six (6) states use definitions that limit the population served by the principal agency only to individuals with mental retardation. In some of these apparently more restrictive states (for instance, Texas) there are other agencies that provide some service and supports to persons with developmental disabilities.